My grandfather joined the Marines in 43, a few months after his first wife died. After completing training he was assigned to the 6th Marine Division and landed on D-Day at Okinawa. He was 35 years old when he waded ashore.
When historians estimate the loss of civilian life at Okinawa there should be adding a footnote or two. First Okinawan culture was still distinct from Japanese culture at the time of the battle. Evidence of this comes from a tale my grandfather told of capturing some Japanese soldiers hiding among the Okinawan populace. He was alerted of their presence by the Okinawans. Second, about half the island was undefended. A sizable portion of Okinawan civilians did not evacuate behind the infamous Surri line and were “captured” by the Marines assigned to take the north end of the island. Keep this in mind when you hear about civilian deaths of about one third of the population. Now extrapolate that to what would have happened on the main islands of Japan, where every inch would have been contested. It is difficult to imagine.
Americans rarely talk about the British contribution to the battle. Many British ships (English, Australian and New Zealand flagged) took part and were attacked by kamikaze along with the America fleet.
Further, Churchill committed the Empire to the invasion of Honshu, Operation Olympia, along with the Americans. Britain estimated they would suffer about 500,000 casualties. This is on top of the estimated 1,000,000 American dead, missing and wounded. Japanese casualties would have been horrific.
The NappyOne
Also, while not that advanced, you had the Japanese nuclear weapon program https://en.wikipedia.org/wiki/Japanese_nuclear_weapon_program